Abstract: The invention relates to an apparatus for the irradiation, in particular UV irradiation, of a liquid. The irradiation takes place in this case under precisely defined and reproducible operating conditions. The apparatus has a rotating cylinder with an upper inflow, a lower outflow and an irradiation unit, the cylinder forming an angle of inclination (&agr;) with the horizontal. The lower outflow also has two connections for passing on the liquid, one connection being closable with the aid of a first control unit if the irradiation of the liquid deviates from the defined conditions.